Driving circuit suppressing peak value of charging current from power supply to capacitive load

ABSTRACT

A buffer circuit, which supplies current to a capacitive load, has a first circuit for reducing the power supply charging current to the capacitive load during switching intervals. The first circuit includes a charge storage device precharged between inverter switching intervals to produce at least a portion of the load charging current during the switching intervals. A second circuit includes a switching element connected between the power supply and the capacitive load to electrically connect the power supply through the second circuit to the load at a selected time in the switching interval to supplement the charging current produced by the charge storage device.

BACKGROUND OF THE INVENTION

The present invention relates to an inverter or switching circuit making use of complementary insulated gate field effect transistors (hereinafter abbreviated as CMOSFET's). More particularly, it relates to an inverter circuit with a capacitive load connected to the output terminal thereof in which a heavy transient current flows from a power supply under a transient condition where an input signal is changed to invert the inverter output.

Since an inverter circuit using CMOSFET's (hereinafter abbreviated as CMOS inverter) consumes little power in the steady state condition, its use in large-scale integrated circuits (thereinafter abbreviated as IC) is desirable. Under the transient condition, however, a considerably large power supply current (hereinafter abbreviated as I_(DD)) flows through the CMOS inverter due to a charging current flowing to a capacitive component of the load. Accordingly, the peak value of I_(DD) in the CMOS inverter is large during the transient period. Therefore, it is necessary to take measures, such as lowering the impedance of power supply wirings to the CMOS inverter to reduce this transient current. But such a measure would deteriorate a valuable feature of the CMOS circuit; namely, that it can use fine wirings because of the low steady state power consumption. This problem becomes more remarkable in a memory IC where a large number of address inverters operate simultaneously. For instance, in a 64K-bit memory, an address input is 16-bits, which means that at least 16 address inverters operate simultaneously in response to 16 address input signals applied in parallel. In that case, I_(DD) is multiplied by a factor of 16, causing an extremely large peak current to flow from the power supply of this memory IC. Consequently, a noise is generated in the power supply lines, causing many faults in the operation of the memory IC.

It has been proposed to reduce the peak current by prolonging the CMOS inverter switching time to thereby gradually charge the load capacitance. However, this method has a drawback that speed-up of the whole IC is prevented because of the prolonged response time of the inverter.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an inverter circuit, in which the peak value of the load capacitance charging current component of the power supply current can be largely reduced while maintaining high-speed operation.

According to one feature of the present invention, there is provided an inverter circuit responsive to an input signal for outputting an inverted signal of the input signal, with a first auxiliary circuit connected to the inverter circuit. The first auxiliary circuit includes an auxiliary capacitor which is charged at a time other than the transient time when the output transfers from the low level to the high level (e.g. when the input signal is "1" (or "0") level), and is discharged during the transient period when the input signal is changed from the low level to the high level. Since the discharge current from the auxiliary capacitor is added to the power supply current from the power supply for charging the capacitive load in the transient period, the peak power supply current during the transient period can be greatly reduced. The power supply current for charging the auxiliary capacitor flows at a time other than the transient period and contributes to the reduced peak current.

According to another feature of the invention, a second auxiliary circuit is added to the inverter circuit in parallel with the first auxiliary circuit to feed a part of the output current of the inverter circuit at least after the discharging current from the auxiliary capacitor starts to flow and favorably after the mentioned discharging current ends during the transient period.

The inverter circuit may comprise a first P-channel type field effect transistor (hereinafter abbreviated as Pch-FET) and a first N-channel type field effect transistor (hereinafter abbreviated as Nch-FET) connected in series between the two power supply terminals, the gates of these FET's being commonly connected to an input terminal, and the common junction point therebetween being used as an output terminal. The first auxiliary circuit may include second, third and fourth Pch-FET's (or Nch-FET's) connected in series between one of the power supply terminals and the output terminal of the inverter circuit with their respective gates connected to the other power supply terminal, the output terminal of the inverter circuit and the input terminal of the inverter circuit, respectively. An auxiliary capacitor is inserted between the common junction point between the second and third FET's and the other power supply terminal. The second auxiliary circuit may include a fifth Pch-FET (or Nch-FET) inserted between the one of the power supply terminals and the output terminal of the inverter circuit and a delay circuit coupled between the input terminal of the inverter circuit and the gate of the fifth FET. The second auxiliary circuit may further include a sixth Pch-FET (or Nch-FET) inserted between the drain of the fifth FET and the output terminal of the inverter circuit and having its gate connected to the input terminal of the inverter circuit. With the first and second auxiliary circuits, the inverter circuit according to this invention can reduce the peak value of the capacitive load charging current component of the power supply current as compared to the prior art inverter circuit, while maintaining a high-speed operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a basic inverter circuit in the prior art making use of CMOSFET's.

FIG. 2 illustrates the waveforms of an input signal φ_(IN), an output signal φ_(OUT), and a charging current component I_(DDL) of the power supply current I_(DD) as functions of time for the inverter circuit shown in FIG. 1,

FIG. 3 is a circuit diagram showing one preferred embodiment of the present invention,

FIG. 4 illustrates the waveforms of the input voltage φ_(IN) the output voltage φ_(OUT) and the charging current component I_(DDL) of the power supply current as functions of time as the inverter circuit of FIG. 3 goes from the "1" level to "0" level.

FIG. 5 illustrates the waveforms of the input voltage φ_(IN), the output voltage φ_(OUT) and the charging current components of the power supply current as functions of time as the inverter circuits of FIGS. 3 and 6 are changed from "0" level to "1" level.

FIG. 6 is a circuit diagram showing another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a prior art inverter circuit comprises an Nch-FET Q₁ and a Pch-FET Q₂ with their drains connected in common to form an output terminal 3. The source of the FET Q₁ is connected to a V_(SS) power supply terminal (in the illustrated example, to the ground terminal) and the source of the FET Q₂ is connected to a V_(DD) power supply terminal 4 (in the illustrated example, V_(DD) represents a positive voltage). The gates of both FET's are connected in common and form an input terminal 2. It is to be noted that a load capacitor C_(L) is present between the output terminal 3 and the ground terminal.

Referring also to FIG. 2, when the input signal φ_(IN) is at "1" level (in the illustrated example, a high voltage V_(DD) level) at a time t_(o), the Nch-FET Q₁ is ON, and hence the output signal φ_(OUT) maintains "0" level (V_(SS) level). Since the Pch-FET Q₂ is OFF at this time, the power supply current I_(DD) does not flow. Subsequently, the input signal φ_(IN) begins to transfer from the "1" level to the "0" level (in the illustrated example the low voltage V_(SS) is the 0 volt level), and when the voltage of the input signal has lowered to V_(DD) -|V_(TP) | (V_(TP) : a threshold voltage of the Pch-FET Q₂) at a time t₁, the Pch-FET Q₂ is turned ON and a power supply current I_(DD) begins to flow. As a result, the voltage of the output signal φ_(OUT) begins to rise, determined by the capability ratio between the Nch-FET Q₁ and the Pch-FET Q₂. As the voltage of the output signal φ_(OUT) rises, the load capacitor C_(L) is driven, and a charging current I_(DDL) flows from the V_(DD) power supply terminal through the Pch-FET Q₂ into the load capacitor C_(L). Therefore, the power supply current I_(DD) which is the sum of the charging current I_(DDL) and a current I_(DDO) flowing through the FET Q₁ to the ground becomes large. Then, the input signal φ_(IN) approaches the "0" level, and when the voltage reaches V_(TN) (a threshold voltage of the Nch-FET Q₁), the Nch-FET Q₁ is turned OFF, and the power supply current component flowing through the Nch-FET Q₁ (I_(DDO)) stops. When the input signal φ_(IN) reaches "0" level at a time t₂, the output signal φ_(OUT) also reaches "1" level, so that the I_(DDL) also stops flowing. Thus the charging current component I_(DDL) has a nearly symmetric waveform with a large peak value. The peak value of the charging current I_(DDL) become larger as the load capacitor C_(L) is increased. As described previously, an important problem to be solved is how best to reduce the peak value of this I_(DDL).

With reference to FIG. 3, the circuit of a preferred embodiment of the present invention comprises a basic inverter circuit 11 which includes a first Pch-FET Q₁₂ and a first Nch-FET Q₁₁ connected in series at an output terminal 17. The source of the Nch-FET Q₁₁ is connected to V_(SS) terminal (the ground terminal in this example) and the source of the Pch-FET Q₁₂ is connected to a V_(DD) power supply terminal 16. The gates of the Nch-FET Q₁₁ and the Pch-FET Q₁₂ are both connected to an input terminal 15. A first auxiliary circuit 12 is added to the inverter circuit 11, which in turn includes second, third and fourth Pch-FET's Q₁₄, Q₁₅ and Q₁₆ connected in series between the V_(DD) power supply terminal 16 and the output terminal 17. The respective gates of Q₁₄, Q₁₅ and Q₁₆ are connected to the input terminal 15, the output terminal 17, and the ground terminal, respectively. The first auxiliary circuit further includes an auxiliary capacitor C_(A) inserted between the common junction point N2 of the Pch-FET's Q₁₅ and Q₁₆ and the ground terminal. A second auxiliary circuit 13 is also added, which includes a fifth Pch-FET Q₁₃ inserted between the V_(DD) power supply terminal 16 and the output terminal 17 and a delay circuit 14 inserted between the input terminal 15 and the gate of Q₁₃. The delay circuit 14 is used to delay the input signal φ_(IN) by a predetermined period. Since precision is not required, the delay circuit 14 can be easily constructed using known techniques.

Now description will be made of the operation of this circuit when the input signal φ_(IN) transfers from "1" level (V_(DD)) to the "0" level (ground), with reference to FIG. 4. Since the basic inverter circuit 11 has the same construction as the circuit in the prior art shown in FIG. 1, its operation is basically the same as that of the prior art circuit.

At first, when the input signal φ_(IN) is at the "1" level (V_(DD)) at a time between t_(o) and t₁₁, the Pch-FET's Q₁₅ and Q₁₆ of the first auxiliary circuit 12 are ON and the Pch-FET Q₁₄ is OFF. Hence the auxiliary capacitor C_(A) is charged via the Pch-FET Q₁₆ by V_(DD) and stores electric charge therein. Accordingly, the charging current I_(DDA) for this capacitor C_(A) flows from the V_(DD) power supply terminal 16 as a component of the power supply current I_(DD), during the period of time t_(o) to t₁₁. At this moment, the Nch-FET Q₁₁ is ON and the Pch-FET's Q₁₂ and Q₁₃ are OFF. Therefore, the output voltage φ_(OUT) at the output terminal 17 is at the "0" level, and the power supply current I_(DDO) through the Nch-FET Q₁₁ and the charging current I_(DDL) for the load capacitor C_(L) are both zero.

Within the period of t₀ to t₁₁, the input signal φ_(IN) starts to fall. When the voltage of the input signal φ_(IN) is lowered to V_(DD) -|V_(TP) | at the time t₁₁, the Pch-FET Q₁₂ and the Pch-FET Q₁₄ are turned ON, and hence a current I_(DDO) through the Pch-FET Q₁₂ and the Nch-FET Q₁₁ and a charging current I_(DDL1) through the Pch-FET Q₁₂ and load capacitor C_(L) for charging the load capacitor C_(L) begin to flow. Furthermore, the electric charge previously stored in the auxiliary capacitor C_(A) begins to discharge as a part of the charging current for the load capacitor C_(L) through the Pch-FET Q₁₄ (this component being represented by I_(DDL2)). Since this discharge current I_(DDL2) is based on the discharge of electric charge that has been preliminarily stored in the auxiliary element C_(A), an increment of the power supply current I_(DD) that is necessary for I.sub. DDL2 after the time t₁₁ is very small. If the conductance of the Pch-FET Q₁₆ is chosen less than about 1/10 of that of the Pch-FET Q₁₂, then the I_(DD) component passing through the Pch-FET Q₁₆ is very small, and therefore it can be neglected. Moreover, if the auxiliary capacitor C_(A) is chosen nearly the same size as the load capacitor C_(L), then about one-half of the charging current I_(DDL) can be obtained from I_(DDL2).

Subsequently, the voltage of the input signal φ_(IN) is further lowered after t₁₁ and accordingly the voltage of the output signal φ_(OUT) rises, and when the output voltage value reaches about V_(DD) /2 at a time t₁₂, the input signal φ'_(IN) delayed by the delay circuit 14 in the second auxiliary circuit is applied to the Pch-FET Q₁₃, so that the Pch-FET Q₁₃ is turned ON and sends out an output current to the output terminal 17 which forms another part of the charging current for the load capacitor C_(L) (this component being called I_(DDL3)). As a result the power supply current I_(DD) now includes the I_(DDL3) component. On the other hand, the potential at the node N₂ which serves as one terminal of the auxiliary capacitor C_(A) becomes nearly the same level as the φ_(OUT) when the voltage of the output signal φ_(out) exceeds V_(DD) /2, and hence the Pch-FET Q₁₅ is turned OFF, so that the charging current I_(DDL2) from the auxiliary capacitor C_(A) is eliminated. Therefore, at this time the charging current for the load capacitor C_(L) is comprised of two components; that is, the I_(DDL1) passing through the Pch-FET Q₁₂ and the I_(DDL3) passing through the Pch-FET Q₁₃. This I_(DDL3) component compensates for the loss of the above-described I_(DDL2) component to promote charging of the load capacitor C_(L) and serves to quickly raise the voltage of the output signal φ_(OUT). Therefore, it is favorable to select the conductance of the Pch-FET Q₁₃ larger than that of the Pch-FET Q₁₂.

Next, when the input signal φ_(IN) approaches the "0" level and its voltage becomes equal to or lower than V_(TN), then the Nch-FET Q₁₁ is turned OFF, and hence the I_(DDO) component passing through the Nch-FET stops. Then, the input signal φ_(IN) reaches the "0" level and the output signal φ_(OUT) reaches the "1" level at a time t₁₃, and as a result, the I_(DDL1) component passing through the Pch-FET Q₁₂ as well as the I_(DDL3) component passing through the Pch-FET Q₁₃ are also eliminated.

As will be apparent from the above description, in the circuit of the illustrated embodiment, the charging current I_(DDL) for the load capacitor C_(L) is formed in such a manner that until the voltage of the output signal φ_(OUT) becomes nearly equal to V_(DD) /2, the charging current is comprised of the I_(DDL1) passing through the Pch-FET Q₁₂ and the discharging current I_(DDL2) of the auxiliary capacitor C_(A) which has been preliminarily charged, and after the output signal φ_(OUT) nearly exceeds V_(DD) /2, the I_(DDL2) is eliminated and instead the I_(DD3) passing through the Pch-FET Q₁₃ is newly added. Consequently, the load capacitor charging current component I_(DDL) of the power supply current of the circuit would flow over the entire region of operation, and its peak value during the period t₁₁ to t₁₃ becomes very small as shown in FIG. 4. The extent of this reduction of the peak value depends upon the design of the first and second auxiliary circuits such as the magnitude of the auxiliary capacitor C_(A) and the conductance of the Pch-FET Q₁₃. However, it is quite easy to reduce the peak value of the charging current to 1/2 or less of the peak value in the prior art inverter circuit.

Furthermore, since those auxiliary charging currents can be subjected to appropriate adjustment by varying the delay characteristics of the delay circuit 14 in the second auxiliary circuit 13 so as to meet the response time of the circuit, there is no need to prolong a response time of the inverter circuit 11, and the response time may be rather shortened by selecting appropriate timing.

Now description will be made of the case where the input signal transfers from the "0" level to the "1" level, with reference to FIG. 5 which shows waveforms of the input signal φ_(IN), the output signal φ_(OUT) and the power supply current I_(DD) for this case.

At first, during the period t₀ to t₂₀, when the input signal φ_(IN) is at the "0" level (ground), the Pch-FET's Q₁₂, Q₁₃, Q₁₄ and Q₁₆ are ON and the Nch-FET Q₁₁ and the Pch-FET Q₁₅ are OFF. Accordingly, the auxiliary capacitor C_(A) is charged, and a power supply charging current I_(DDA) flows.

Next, the input signal φ_(IN) starts to rise at a time t₂₀. When it rises up to V_(TN) at a time t₂₁, the Nch-FET Q₁₁ is turned ON and the discharging current of the load capacitor C_(L) begins to flow through the Nch-FET Q₁₁. Furthermore the power supply current I_(DDO) flows through the Pch-FET Q₁₂ and the Nch-FET Q₁₁, and the power supply current I_(DDO) ' flows through the Pch-FET Q₁₃ and the Nch-FET Q₁₁. At this moment, since the Pch-FET Q₁₅ is kept OFF, only the I_(DDA) flows through the first auxiliary circuit.

Subsequently, when the input signal φ_(IN) reaches V_(DD) -|V_(TP) |, at a time t₂₃, the Pch-FET Q₁₃ is turned OFF and the I_(DDO) stops flowing, but since the voltage of the delayed signal φ_(IN) for the input signal φ_(IN) which is a driving voltage for the Pch-FET Q₁₃ does not rise as shown in FIG. 5, the Pch-FET Q₁₃ is still kept ON, and so the I_(DDO) ' continues to flow. Thereafter when the φ'_(IN) becomes V_(DD) -V_(TP) at a time t₂₄, the Pch-FET Q₁₃ is turned OFF and the I_(DDO) ' stops flowing. The input signal φ_(IN) reaches the "1" level, and the output signal φ_(OUT) reaches the "0" level.

In other words, in the transient period when the input signal φ_(IN) transfers from the "0" level to the "1" level, the I_(DDO) ' passing through the Pch-FET Q₁₃ is added to the power supply current I_(DDO) which flows together with the discharge current of the load capacitor C_(L) in the circuit known in the prior art, and therefore, the overall power supply current I'_(DD) takes the form shown in FIG. 5. As described, the circuit of the illustrated embodiment of FIG. 3 has a problem that although the peak value of the load capacitor charging current component I_(DDL) of the power supply current I_(DD) can be greatly reduced when the input signal φ_(IN) transfers from the "1" level to the "0" level, the power supply current I_(DDO) ' caused by the second auxiliary circuit is added to the power supply current when the input signal φ_(IN) transfers from the "0" level to the "1" level.

The circuit of another preferred embodiment of the present invention shown in FIG. 6 solves the above-mentioned problem. The only difference from the circuit shown in FIG. 3 and described previously resides in that the second auxiliary circuit 13' includes another Pch-FET Q₁₇ inserted between the drain of the Pch-FET Q₁₃ and the output terminal 17 and having its gate connected to the input terminal 15. In this circuit of the modified embodiment, the Pch-FET Q₁₇ is turned OFF in response to the input signal φ_(IN) and the I_(DDO) ' also ceases to flow at the same time when I_(DDO) ceases to flow through Q₁₂. Therefore, the I_(DD) in this modified embodiment becomes small as indicated by I"_(DD) in FIG. 5.

It is to be noted that in the above-described embodiments, the conductivity type of the respective FET's can be changed if necessary. For instance, in place of the Pch-FET an Nch-FET can be used for the FET Q₁₄, and an inverted input signal φ_(IN) would then be applied to the gate of this FET. The Pch-FET's and Nch-FET's may be replaced by Nch-FET's and Pch-FET's, respectively, with the terminals of the power supply voltage being reversely connected. 

What is claimed is:
 1. An inverter circuit comprising a first field effect transistor of one conductivity type, a second field effect transistor of the opposite conductivity type connected with said first transistor in series between two terminals of a power supply, an input terminal coupled to the gates of said first and second transistors, an output terminal coupled to the connection point of said first and second transistors, a capacitive load coupled to said output terminal, a capacitor, first means coupled to said capacitor and to one of said two terminals of the power supply for charging said capacitor at a time other than a transient time when a potential at said output terminal transfers from a first level to a second level, and a second means coupled to said capacitor and said capacitive load for charging said capacitive load with a discharging current from said capacitor during said transient time.
 2. The inverter of claim 1, further comprising a third means coupled to said one terminal of the power supply and said capacitive load for charging said capacitive load with a current from said power supply during said transient time after the discharging current from said capacitor starts to flow.
 3. An inverter as claimed in claim 1, in which said first means includes a first switch transistor connected between said one terminal of the power supply and said capacitor and said second means includes a second and a third switch transistors connected in series between a connection point of said first switch transistor and said capacitor and said capacitive load, said second switch transistor being controlled by the voltage at said output terminal, and said third switch transistor being controlled by an input signal at said input terminal.
 4. An inverter as claimed in claim 2, in which said third means includes a delay circuit for delaying an input signal at said input terminal and a fourth switch transistor connected between said one terminal of the power supply and said output terminal.
 5. An inverter comprising an inverter circuit responsive to an input signal for producing an output signal that is inverted relative to the input signal, a first auxiliary circuit and a second auxiliary circuit connected in parallel between an output terminal of said inverter circuit and a power supply, respectively, said first auxiliary circuit including an auxiliary capacitor and including means to discharge electric charge stored in said auxiliary capacitor during a transient period where the output of the inverter goes from a "0" level to a "1" level, said second auxiliary circuit including means for applying a current to the output terminal of said inverter circuit in a predetermined period during said transient period.
 6. An inverter as claimed in claim 5, in which said inverter circuit includes a first P-channel type field effect transistor and a first N-channel type field effect transistor connected in series and inserted between the power supply and a circuit ground terminal, the gates of said two transistors being connected in common to form an input terminal, a common junction point between said two transistors being coupled to an output terminal, said first auxiliary circuit including second, third and fourth P-channel type field effect transistors connected in series with each other and having their gates connected to the ground terminal, said output terminal and said input terminal, respectively, and an auxiliary capacitor inserted between the common junction point of said second and third P-channel type field effect transistor and the ground terminal, said second auxiliary circuit including a delay circuit and a fifth P-channel type field effect transistor having its gate connected through the delay circuit to said input terminal.
 7. An inverter as claimed in claim 6, in which said second auxiliary circuit includes a sixth P-channel type field effect transistor inserted between said fifth P-channel type field effect transistor and said output terminal and having its gate connected to said input terminal.
 8. In an inverter circuit for applying an output signal to a load, first means responsive to an input signal for applying a first charging current to said load during selected switching intervals of said inverter circuit, a charge storage device, and second means responsive to said input signal for charging said charge storage device between said switching intervals of said inverter circuit and for causing said charge storage device to discharge into said load during said selected inverter circuit switching intervals.
 9. In the inverter circuit of claim 8, further including a third means for applying a second charging current to said load during selected periods within inverter switching intervals, said third means including a delay circuit responsive to said input signal and a switching means responsive to said delay circuit for applying said second charging current a predetermined time after the input signal begins to switch from a first level causing said output signal to assume a relatively low voltage state to a second level causing said output signal to assume a relatively high voltage state.
 10. A circuit for driving a capacitive load, comprising first means responsive to an input signal for charging and discharging said capacitive load, a capacitor, second means coupled between said capacitor and a power supply terminal for charging said capacitor before said first means charges said capacitive load, and third means coupled between said capacitor and said capacitive load for charging said capacitive load by a discharge current from said capacitor when said first means charges said capacitive load.
 11. The circuit as claimed in claim 10, further comprising fourth means coupled between said power supply terminal and said capacitive load for charging said capacitive load after said discharge current from said capacitor starts to flow. 